Apparatus are commercially available for measuring the transmission characteristics of an optical fiber, especially distortions in the waveform of light being transmitted through the fiber. An example of such an apparatus is hereunder described with reference to FIG. 6. As shown, a laser diode LD is driven by a laser diode drive unit to emit pulsed light. This pulsed light is incident upon one end of an optical fiber under test F and the light emerging from the other end of the fiber is converted to an electrical signal by a photodiode PD. The output of PD is amplified by an amplifier Amp for monitoring on an oscilloscope M or other suitable means. A rough estimation of the characteristics of the fiber under test can be obtained by observing the waveform on the CRT. If more accurate data are needed, the output of Amp may be digitized by an A-D converter A/D and stored in a memory for future use in performing various analyses.
The maximum time resolution that can be achieved by the system shown above is on the order of 10.sup.2 picoseconds and is not high enough to resolve small waveform distortions. With a view to compensating for the low time resolution of this system by magnifying waveform distortions, it is recommended that an optical fiber under test should have a length of about 1 km. Such a method, however, is incapable of accurate measurement because even a distortion that occurs in part of the 1 km long fiber is only picked up after being averaged over the length of 1 km.